Cellular communication systems are currently being developed and improved for machine type communication (MTC). MTC is characterized by lower demands on data rates than for example mobile broadband, but with higher requirements on e.g. low cost device design, better coverage, and ability to operate for years on batteries without charging or replacing the batteries. Currently, 3GPP is standardizing a feature called Narrowband Internet of Things (NB-IoT) for satisfying all the requirements put forward by MTC type applications, while coexisting with the legacy Long Term Evolution (LTE) radio access technology. At 3GPP RAN #70 meeting, a new work item named Narrowband IoT (NB-IoT) was approved. See RP-152284, “New Work Item: Narrowband IoT (NB-IoT),” sources Huawei and HiSilicon, RAN #70. The objective is to specify a radio access for cellular internet of things that addresses improved indoor coverage, support for massive number of low throughput devices, low delay sensitivity, ultra-low device cost, low device power consumption and (optimized) network architecture.
For NB-IoT, three different operation modes are defined, i.e., standalone, guard-band, and in-band. In standalone mode, the NB-IoT system is operated in a dedicated frequency band, e.g., refarming one or more Global Systems for Mobile communication (GSM) channels. For in-band operation, the NB-IoT system can be placed inside the frequency bands used by the current LTE system, while in the guard-band mode, the NB-IoT system can be placed in the guard band used by the current LTE system. The NB-IoT has a system bandwidth of 180 kHz, i.e. substantially smaller than the LTE system bandwidth which is in the range from 1.4 MHz to 20 MHz.
For the NB-IoT system uplink, both single-tone and multi-tone operations are defined. For single-tone uplink, both 3.75 kHz and 15 kHz subcarrier spacings are supported. From RP-152284, it is stated “Single tone transmissions are supported. Two numerologies should be configurable by the network for single-tone transmission: 3.75 kHz and 15 kHz, A cyclic prefix is inserted. Frequency domain sinc pulse-shaping in the physical layer description. Multi-tone transmissions are supported, based on Single Carrier Frequency Division Multiple Access (SC-FDMA) with 15 kHz UL subcarrier spacing.” In addition, for multi-tone uplink, the numbers of subcarriers that can be used are currently agreed to be 3, 6, and 12. See R1-16xxxx, RAN1 agreements for Rel-13 NB-IOT, source, WI rapporteur (Ericsson), 3GPP TSG-RAN WG1 Meeting #84, St. Julian's, Malta, Feb. 15-19, 2016. As the NB-IoT system bandwidth is 180 kHz, which is the same size as 1 physical resource block (PRB) in the current LTE system, there are at most 12 subcarriers.
For the case of 12 subcarriers allocated to an NB-IoT device for its uplink transmission, it can re-use the uplink reference symbols designed for LTE. But as the minimum resource allocation granularity of the LTE system is 1 PRB, there are no shorter reference symbol sequences than length 12 defined. Therefore, for NB-IoT devices that are allocated 3 or 6 subcarriers for their uplink transmission, there are no reference symbol sequences defined.